This disclosure relates, in various embodiments, to processes for purifying 2-phenyl-3,3-bis(4-hydroxyphenyl)phthalimidine.
Phenolphthalein has been used as an aromatic dihydroxy compound to synthesize monomers for preparing polycarbonates, which are generally characterized with excellent clarity, excellent ductility, and high glass transition temperatures. Certain derivatives of phenolphthalein have also been used as aromatic dihydroxy compounds to synthesize monomers to prepare polycarbonate resins as well as polyarylate resins. In particular, 2-phenyl-3,3-bis(hydroxyphenyl)phthalimidine (“PPPBP”) is useful as a monomer for polycarbonate resins.
para,para-PPPBP (“p,p-PPPBP”) can be synthesized by refluxing phenolphthalein and aniline hydrochloride in aniline for 6 hours, followed by recrystallization from ethanol. p,p-PPPBP has the chemical structure of Formula (I):

As is evident, the “p,p-” designation is used because both hydroxyls are in the p-position. During this synthesis, undesired side products and impurities are created. Two such undesired byproducts are ortho,para-PPPBP (“o,p-PPPBP”) and aminophenone. o,p-PPPBP has the chemical structure of Formula (II) and aminophenone has the chemical structure of Formula (III):
wherein R1, R2, R3, and R4 are independently —OH or —NH2; and at least one of R3 and R4 is —NH2. These byproducts arise as a result of the synthesis method described above. Although the structures of Formulas (II) and (II) encompass several different compounds, they will be treated as one for the purposes of this disclosure.
Other impurities include residual levels of phenolphthalein and other phenolphthalein by-products. Impurities affect polymer properties considerably. They can hinder polymerization and result in low weight average molecular weight polycarbonates of, for example, less than about 22,000 Daltons for melt polymerization and less than about 50,000 Daltons for an interfacial polymerization that exhibit undesirable physical properties, such as increased brittleness, that is, poor ductility properties. Furthermore, they affect the transparency of the polymer product by producing discoloration. A major objective of such polycarbonates is transparency.
Because impurities affect the final polymer product, p,p-PPPBP must be purified after synthesis. The purification process, also known as the decoloring process, reduces and/or removes these impurities from the intermediate polymer mixture to obtain the final desired product. Monomer-grade p,p-PPPBP should contain impurities at a level of less than 15 parts per million (ppm).
In present decoloring processes, the intermediate polymer mixture, containing a mixture of p,p-PPPBP, phenolphthalein, o,p-PPPBP, aminophenone, and other byproducts, is dissolved in an aqueous inorganic base. This solution is then treated with an adsorbent, usually powder activated carbon (“PAC”), to remove the impurities. After treatment with PAC, the resulting mixture is then filtered to obtain the purified p,p-PPPBP in aqueous solution. This process is repeated several times to achieve the desired purity level.
This process has several disadvantages. First, the PAC usually absorbs between 7-8% of the desired p,p-PPPBP product. Because PAC is difficult to regenerate, it is usually discarded after one use and the absorbed product is consequently lost. In addition, the cost of the PAC, which is discarded, is relatively high due to the need for a specific grade of PAC. Finally, because the PAC is of small diameter (to increase surface area for reaction), it is difficult to filter the PAC out of the mixture to obtain the p,p-PPPBP in aqueous solution.
There is a continuing need for the removal and/or reduction of impurities from p,p-PPPBP. Such a purification process should also be cost-effective and feasible on a large scale.